intern-dev-api uses different file separators on MacOS and Windows.

When I run a build from one of the Intern repos on MacOS, intern-dev-api writes file paths to api.json using forward slashes ("/") as separators.  When I run the same build on Windows, all of the file paths are updated to use double backslashed ("\\\\") as separators.

It should use forward slashes always on all platforms.